Transmission system



June 18, 1935. c. w. HANSELL 2,005,034

TRANSMISSION SYSTEM Original Filed July 20, 1927 INVENTOR CLARENCE W. HANSELL BY 7%;4M

ATTORNEY Patented June 18, 1935 PATENT OFFICE TRANSMISSION SYSTEM -Cla.rence W. Hansell, Port .lefierson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Original application July 20, 1927, Serial No. 207,114. Divided and this application March .9, 1932, Serial No. 597,754. In Germany June 3 Claims.

5 plication Serial No. 207,114, filed July 20, 1927 and which has issued as United States Patent No. 1,874,978, dated August 30, 1932, and in particular is directed towards Figure 3 of the parent application.

Transmission stations of large size are usually located in suburban localities, whereas the signalling preferably is accomplished from an urban ofiice. Such an arrangement becomes especially necessary when the transmitting and receiving 15 stations for one city are widely separated and both have land lines leading to a signalling office; The keying means at the office heretofore have controlled, through land lines, a direct current which keys the transmitter through an elaborate system of relays. Consequently the maximum possible keying speed has been limited by the time characteristics of the land line and the rapidity of response of the relays. It is an object of my invention to increase the maximum keying speed by decreasing the effect of the land line, and this I find is possible by using alternating current energy on the line in place of direct current energy.

It is a further object of my invention to simplify transmission apparatus and to increase keying speed by eliminating the relay system previously employed, which I do by utilizing the alternating energy from the land line to modulate high frequency transmission energy and transmitting only a desired portion of the resulting modulated energy. Preferably a carrier suppression modu lator is employed to eliminate the carrier component of the modulated energy, and then one of the two resulting side frequencies is selected by a filter and is suitably amplified for transmission. With this arrangement there is no transmitter output when the land line is not keyed.

This invention allows multiplex transmission over a single land line which may readily be accomplished by establishing a desired number of signal channels of different frequency ranges on the land line, and signalling either by energy control or frequency control, in each of these channels. This invention finds it desirable to have a carrier current or relatively high frequency land line, but this is not essential, and in order to employ the usual land lines the frequency channels which may be employed should all stay within a range of several thousand cycles per second. A short wave or very high frequency transmission is favored, one consequence'oflow land line frequency will .be a very small separation between the upper and lower side band frequencies, thereby making it difficult to successfully filter one from the other. To overcome this difficulty is a still further object of my invention and some or all of various expedients may be employed. It is preferred to use a special'filter such as a crystal filter which is sufficiently highly selective to discriminate between upper and lower side frequencies even though these are separated by a relatively small amount. If desired, frequency multiplication may be utilized before transmission. In such case the carrier frequency at the time of modulation may be much lower,

and therefore for a given modulating frequency,

the percentage difference between the upper and lower side frequencies is increased. I

It is difficult to operate power amplifiers at very high frequencies under the control of very small amounts of controlling energy, and many of these difiiculties are overcome by using, instead of straight amplification, frequency multiplication together with amplification, which presents an additional advantage concomitant with frequency multiplication.

The invention is more particularly described in the following specification, which is accompanied by a single drawing which shows a wiring diagram for a transmitter employing successive modulation, crystal side band separation, and frequency multiplication and amplification.

Referring tothe single drawing, there is shown a source of alternating current energy 2 controlled by a key A, the energy from which is conducted over a land line ii toapparatus at the transmitting station. There is a local oscillator N3, the energy from which is applied symmetrically through radio frequency condensers 9 to a push-pull modulator stage 20. The carrier component of the modulated energy is suppressed because of phase opposition in the output circuit of the modulator. The upper and lower side frequencies are separated by a filter Z2 and one of the two side frequencies, preferably the upper, is used to modulate energy of the carrier frequency, generated in an oscillator 8 through the agency of a carrier suppression modulator I ll.

The side frequencies are separated in accord-' ance with this invention by a crystal filter which is utilized for close frequency selection, which may be followed by a power amplifier, or fre- I quency multiplier, or frequency multiplier amplifier, and then radiated. With such an arrangement the frequency separation of the side frequencies is greatly increased. As a random example, if the line frequency is 1,000 and the intermediate frequency (oscillator I8) 19,000, then the filter 22 must discriminate between 20,000 and 18,000, which is quite feasible, and then if the carrier frequency is 1,980,000 the crystal filter, which is herein designated as 30, must discriminate between 2,000,000 and 1,960,000. Even this is diff cult, but it is much more practicable than a separation of 2,000,000 and 1,998,000, as if the line frequency directly modulated the high frequency carrier.

The separation of the side frequencies is accomplished by a series connected'piezo-electric crystal 30, having a resonant frequency equal to the desired side frequency. The natural capacitance of the crystal and crystal holder is neutralized by a neutralizing condenser 32, as is described more in detail in my copending application for a crystal filter, filed July 7, 1927, Serial No. 203,901.

.To allow ,keying by frequency change, multiplexing, and frequency wobbling, it is desirable that the filters used be adapted to separate side bands, rather than side frequencies, Thus, the filter 22 preferably is made a high pass filter With, cut-off about at the carrier frequency. A crystalfilter is apt to be too sharply selective for use with side bands instead of side frequencies, and therefore its frequency range is broadened by-the use of additional crystals 34 and 36, having adjacent but sufliciently different resonant frequencies, and the use of a parallel resonant circuit .38, all as has been describedin my copending application already referred to.

The output from the crystal filter is led to a frequency multiplier stage 40, followed by a power amplifier M, the output from which is led to radiating means Hi. It is to be understood that any suitable number of successive stages of modulation maybe used, and that the stage 40 may bea simple amplifier stage, but that on the othenhand the amplifier ,14 may be a frequency multiplier and amplifier, such as has been disclcsedin my copending application, Serial No. 177,505, filed March 23rd, 1927.

Having thus described my invention, what I claim is: r v

1. In a transmitting signalling system, the

- combination with a pair ofpush-pull connected electron discharge devices having an inductance coil connecting corresponding anodes of the devices together, of a neutralized piezo-electric crystal selective band pass filter comprising a plurality of crystals having difi'erent resonance frequencies effectively connected in parallel, a connection fromone terminal ofsaid crystal band filter to one end of said inductance, and a connection from the other terminal of said crystal band filter to the other end of said inductance coil, a condenser in said last connection, and a parallel tuned output circuit connected between the midpoint of said inductance and said other terminal of said crystal band filter, whereby said crystal filter enables the passage of a selected group of frequencies for subsequent utilization by said systern.

2. A communication system for transmitting keyed modulated energy having, in combination, a pair of push-pull connected electron discharge devices for combining oscillations of different frequencies and Whose corresponding anodeflelectrodes are connectedtogetherby an'inductance coil, said, coilco nprising the input of a balanced bridge circuit, one leg of which comprises apiezoelectricfiltena vsecondleg of which comprises a variable condenser adjacent said piezo-electric filter, and the othertwo legs of which constitute said inductance coil, and an output circuit conf nected between the midpoint of said inductance coil and the junction of said piezo-electricfilter and variable condenser, whereby said filter enables the passageof selected energy for utilizationby said system. v

3. A transmitting signalling system having, in

combination, a p'air of push-pull connected elecput circuit connected betweenthe midpointtof said inductance and the junction-of said piezoelectric filter and variable condenser, whereby said filter enables the passage of selected energy for subsequent transmission by said system.

CLARENCE W. HANSELL. 

